1. Field of the Invention
The present invention relates to a particulate magnetic recording medium, a magnetic recording device comprising the same, and a magnetic recording method employing the same. More particularly, the present invention relates to a particulate magnetic recording medium that can be applied to heat-assisted recording despite having poorer heat resistance than a hard disk drive (HDD), a magnetic recording device comprising the same, and a magnetic recording method employing the same.
2. Discussion of the Background
As the quantity of information being recorded has increased, constantly higher density recording has been demanded of magnetic recording media. A reduction in the particle size of magnetic material is required to achieve even higher density recording. However, when the size of the particles in the magnetic material is decreased, the energy (magnetic energy) maintaining the direction of magnetization of the magnetic material cannot readily counter the thermal energy, and so-called thermal fluctuation ends up compromising recording retention. The phenomenon where magnetic energy is overcome by thermal energy and the recording is erased can no longer be ignored.
This point will be described. “KuV/kT” is known as an index relating to the thermal stability of magnetization. Ku is the anisotropic constant of a magnetic material. V (also given as “Vact”, hereinafter) is the particle volume (activation volume). The Boltzmann constant is denoted by k. T denotes the absolute temperature. The effect of thermal fluctuation can be inhibited by increasing the magnetic energy KuV relative to the thermal energy kT. However, it is better for the particle volume V—that is the particle size of the magnetic material—to be reduced to achieve higher density recording. Since the above magnetic energy is the product of Ku and V, it suffices to increase Ku to increase the magnetization energy when V is in the low range. However, the switching field of magnetic material with high magnetocrystalline anisotropy increases, resulting in high coercive force, and a large external magnetic field becomes necessary for recording, compromising recording properties. That is, the higher the Ku of the magnetic particles, the more difficult it becomes to write information.
As stated above, it is extremely difficult to achieve the three characteristics of higher density recording, thermal stability, and ease of writing. This is referred to as the magnetic recording trilemma, and is becoming a major issue in the pursuit of ever higher recording densities.
In recent years, in the field of hard disk drives (HDD), one means of resolving the trilemma has been proposed in the form of a recording system (heat-assisted recording) in which the recording portion of the magnetic layer is heated by the magnetic head to lower the coercive force Hc, ensuring ease of writing.
Japanese Unexamined Patent Publication (KOKAI) No. 2008-60293 or English language family member US2008/057352A1 and U.S. Pat. No. 7,781,082, which are expressly incorporated herein by reference in their entirety, discloses controlling the coercive force of the magnetic material through compositional adjustment without heat-assisted recording, in light of the practical difficulties associated with heat-assisted recording.
Additionally, Japanese Unexamined Patent Publication (KOKAI) No. 2003-77115, which is expressly incorporated herein by reference in its entirety, proposes applying heat-assisted recording to particulate magnetic recording media.
As set forth above, Japanese Unexamined Patent Publication (KOKAI) No. 2003-77115 proposes recording on particulate magnetic recording media by heat-assisted recording. However, in reality, it is currently considered impossible in this field for particulate magnetic recording media for heat-assisted recording to attain the level of practical use. That is because in a particulate magnetic recording medium, the support and various layers are comprised of organic materials with poorer thermal resistance than the materials constituting the HDD (inorganic materials, metal, glass, and the like). They are thus thought incapable of withstanding the heat during heat-assisted recording.
Since particulate magnetic recording media can afford better chemical stability than vapor deposition-type magnetic recording media such as HDDs, they are useful magnetic recording media for high-capacity data storage in the recording of information with high long-term reliability. Were it possible to apply heat-assisted recording to such useful particulate magnetic recording media, the trilemma could be resolved and even higher capacity and higher density recording could be achieved.